Abstract

Deontic paradoxes revealed that formalizing conditional obligations and handling different types of detachment are the essential issues in the study of deontic logic. According to their positions regarding principles of detachment, systems of deontic logic can be categorized into systems that allow only factual detachment, systems that allow only deontic detachment and systems that allow both detachments. This study showed that neither a deontic detachment system nor a factual detachment system could justify its discrimination against the detachment rule that it discards. Systems that allow both detachments typically employ temporal restrictions on normative operators. This study challenged the necessity and sufficiency of tensified deontic logics in formalizing conditional obligations and in handling the related detachment principles. ;The strong prima facie nature of moral reasoning indicates that a defeasible logic might provide the logical devices that deontic logic needs. This study incorporated Donald Nute's defeasible logic into deontic logic. Deontic logic with defeasible detachment has advantages over both deontic detachment systems and factual detachment systems, as well as tensified deontic logics. The outstanding feature of DLDD is that it sanctions both deontic and factual detachments yet allows both of them only defeasibly. In this way, DLDD offers a satisfactory resolution to the most important deontic paradoxes. ;An automated reasoner with defeasible reasoning provides us with an efficient and accurate means to implement and test our logical thinking. DLDD is implemented in the programming language d-Prolog. The implementation of the solutions to deontic paradoxes in d-Prolog can also be seen as an effort to contribute to the interdisciplinary study of applying deontic logic to computer science